event sourcing overview

Event sourcing is a software development pattern that allows you to store the history of your application's state changes as a sequence of events. This approach provides several benefits, including audit-friendliness, replay-friendliness, and a more explicit representation of business logic.

Key Concepts

• Events: These are immutable objects that represent a specific change to the system's state. They typically contain metadata such as timestamps, event IDs, and user information.
• Event Store: This is the storage mechanism for events, which can be implemented using various technologies like databases or message queues.
• Aggregate Root: The aggregate root is the central entity in your domain that manages its own state changes by applying events to itself.

Benefits

Audit-friendliness

With event sourcing, you have a complete record of all changes made to the system. This allows for easy auditing and compliance with regulatory requirements.

// Example audit log entry
{
  "id": "123",
  "timestamp": "2022-01-01T12:00:00Z",
  "event_type": "ORDER_CREATED",
  "data": {
    "order_id": "ORD-001"
  }
}

Replay-friendliness

Since events are stored, you can replay them to recreate the system's state at any point in time. This is useful for testing, debugging, and even disaster recovery.

// Replaying an order created event
eventBus.publish(new OrderCreatedEvent(
  orderId: "ORD-001",
  userId: "USR-001"
));

Complex to operate

While event sourcing offers many benefits, it requires a more complex infrastructure setup compared to traditional state-based approaches. This includes managing the event store, handling event serialization and deserialization, and implementing aggregate roots.

Implementing Event Sourcing in Apiary

To implement event sourcing in Apiary, you'll need to:

1. Define your events as immutable objects.
2. Create an event store using a suitable technology (e.g., database or message queue).
3. Implement aggregate roots that manage their own state changes by applying events.

Here's an example of defining an OrderCreatedEvent and implementing an OrderAggregateRoot:

// Event definition
public class OrderCreatedEvent {
  public string orderId { get; }
  public string userId { get; }

  public OrderCreatedEvent(string orderId, string userId) {
    this.orderId = orderId;
    this.userId = userId;
  }
}

// Aggregate root implementation
public class OrderAggregateRoot : AggregateRoot<OrderState> {
  private readonly IEventStore _eventStore;

  public OrderAggregateRoot(IEventStore eventStore) {
    _eventStore = eventStore;
  }

  public void CreateOrder(string orderId, string userId) {
    var orderCreatedEvent = new OrderCreatedEvent(orderId, userId);
    _eventStore.SaveEvent(orderCreatedEvent);

    // Apply the event to update internal state
    State = new OrderState { Id = orderId };
  }
}

Conclusion

Event sourcing is a powerful pattern that provides audit-friendliness and replay-friendliness. However, it requires a more complex infrastructure setup compared to traditional approaches. By implementing event sourcing in Apiary, you'll be able to store the history of your application's state changes as a sequence of events.

Related/Sources

• Event Sourcing
• Apiary Documentation: Event Sourcing
• CQRS and Event Sourcing by Udi Dahan